if __name__ == '__main__':
    txt = "hello world"
    letters = []
    for i in range(65, 91):
        letters.append(chr(i))

    # tuple元组
    # 注意构造包含0和1个元素的元组的特殊语法规则
    tup1 = tuple()  # 空元组
    tup2 = tuple('1', )  # 一个元素，需要在元素后添加逗号

    try:
        s = input("input a string: ")
        def pow2(s):
            print(s*2)
        pow2(s)
    except TypeError:
        print("input error")
